Qsqa197

If the same energy input, Qs. were consumed in eleclric resistance heating or in a gas or oil furnace the heat available for heating would simply be Qs. Therefore, use of the heat pump has increased the heat available for heating to Qs+Qa- Depending on the conditions, the increase in effective heating available may range front 40 to 100 per cent.

Of course the increase in effective heating has been gained at the expense of a considerable involvement in machinery. The capital cost of a heat pump system would inevitably be much greater than a simple furnace. As fuel costs increase, the economy of heat pumps is enhanced.

Fortunately the same machinery used in the above heat pump for heating may also be deployed as the refrigerator cooling unit in an air conditioning system for summer use.

The machinery works in exactly the same way as described above. The only difference, as illustrated in Fig. 19.4, is that now the expansion-space heat exchanger is connected to the conditioned-space heat exchanger so that heat is absorbed from the building at a temperature less than the ambient value. Similarly the compression-space heat exchanger is linked with the heat exchanger of the thermal source (now acting as a thermal dump). The lake which provides heat for space heating in winter may also be used as the thermal reservoir for cooling applications in the summer.

This reversal of the role of the heat exchanger between winter and summer operation may be effected by simply reversing the direction of rotation of the Stirling engine. This would be a simple matter indeed, if the Stirling engine were driven by an electric motor. In the Stirling-Stirling arrangement a reversal in the direction of rotation might he more difficult to achieve.

'lire preferred method for reversing the role of the heat exchangers would likely be some form of flow switching, probably of the intermediate fluids connecting the conditioned space and the thermal source/dump reservoir and the engine.

An early attempt to develop a duplex Stirling engine cooling unit was made by Walker (1968a) under contract to the British Ministry of

Technology. Martini < 1975a) has well outlined some of the principal considerations in a duplex Stirling scheme. Similarly Benson (1978) has outlined some developments in progress at FRG Inc.. San Francisco. Beale, in Chapter 11, refers to a prototype unit operated at Sunpower Inc., Athens, Ohio. Other developments are known to be in progress and commercial application appears likely in the future.

One of the principal advantages of the duplex Stirling arrangement is the use of a common fluid for the combined Stirling engines. This can be most attractive in free-piston engines where the use of a common fluid greatly relaxes the sealing arrangements.

Furthermore, in free-piston engines the use of gas bearings employing the working fluid as the bearing media eliminates the problem of oil or grease lubricants contaminating the working space. Finally, free-piston engines are or can be made self-starting. This is a most attractive feature for solar-powered air-conditioning units operating intermittently. They produce a cooling effect only when the sun is shining enough to induce the engine to start, a rare and happy combination of cause and effect.

STIKI-JNG/RANKINE-CYCLE HEAT PUMPS

Most development work presently in progress is directed to the use of Stirling engines as prime movers driving Rankine-cycle heat pumps. This is despite the attractions of a common working fluid in the duplex Stirling heat pump outlined above. The reason for this is probably related to the historical factors which have led to the separate development of Stirling-cycle prime movers and Rankine-cycle heat pump and refrigeration systems, usually driven by electric motors. Furthermore, there is a profound lack of awareness in the engineering community about the capability of Stirling engines for cooling purposes. All the established applications are at the cryogenic temperature level. The prospects for Stirling-engine refrigerating engines at relatively high temperature levels are rarely addressed in the literature.

The Stirling/Rankine-cycle heat pump is illustrated diagramatically in Fig. 19.5. The system consists of a Beale-type single-cylinder free-piston Stirling engine. The work developed in the engine is used to drive the compressor of a Rankine-cycle refrigeration circuit. Typically this would use a Freon working fluid and consist basically of four elements, a compressor increasing the pressure from p, to p,„ a J-T (Joule-Thompson) valve where the fluid expands from p„ to p, and two heat exchangers where the fluid changes phase (ai evaporating from liquid to vapour in absorbing heat at low pressure and low temperature and (b) condensing gas to liquid at high temperature and high pressure.

The system illustrated in Fig. 19.5 is the system used in the Beale Sunpower engine where an inertia compressor is used in the Freon circuit.

The inertia compressor consists of a heavy mass, which remains substantially fixed in space, within an enclosure rigidly connected to the engine piston and which oscillates along the axis of the fixed mass so that fluid is pumped alternately from the left and right hand sides of the pump.

The cooler of the Stirling engine and the condenser of the Kankine circuit both transfer heat to the heating system of the building. Heat is absorbed from a thermal source at ambient temperature in the evaporator of the Rankine circuit.

Energy to drive the system is provided by burning fuel with air in a combustion chamber and heat is transferred in the heater to the working fluid of the Stirling engine. Heat rejected in the cooler of the engine can be included in the output of the space heater. Further output may be obtained by another heat exchanger downstream of the air preheater on the exhaust side.

In all this utilization of 'waste' it must be recalled that it is advantageous from the power and efficiency aspect to operated a Stirling engine at a low cooler temperature, probably below the temperature at which the space heating might be required. Furthermore successive cooling of the exhaust stream can be unproductive as the temperature decreases towards ambient. Overcooling can result in the condensation of combustion products that may be corrosive or lead to the build-up of surfacc deposits that interfere with the flow.

Substantial work has been carried out by Sunpower Inc. on the development of gas-fired free-piston Stirling engines of the Beale type used for driving the inertia Freon compressors of Rankine-cycle refrigeration and heat pump systems. This work was sponsored by the American Gas Association and was variously reported by Beale et al (1975). The work centred on the development of a Beale single-cylinder free-piston engine of 3 kW capacity in combination with an inertia pump supplied by the Eaton Corp.

After proceeding through successive generations of prototype machines this development was then redirected by the sponsor, A.G.A.. to the General Electric Space Division for pre-production prototype development. Recent work on the G.E. Stirling-engine gas-fired heat pump has been reviewed by Auxer (1977), Richards and Auxer (1978), Marusak and Chiu (1978).

Fortunately the years of experience accumulated at Sunpower were not dissipated by the surprising change in direction by A.G.A. A major U.S. oil company recognizing the inherent potential of the thermally-activated heat pump rapidly assumed financial responsibility for the programme of prototype development. Public disclosure of this program has not yet been made.

It is understood that a major part (one-third) of the total Philips effort on Stirling engines at Eindhoven was, in 1978, directed to the development of small Stirling engines for inclusion as the prime movers in thermally-activated heat pump systems. It is further understood the engine is of the two-cylinder two-piston Rider form of about 10 horsepower for use in domestic heat pump systems. This hearsay should be confirmed by reference to the Philips Naturdig Laboraiorium or by reference to publicity or informational material that may be published subsequent to this work.

Benson (1978), in a general review of work on advanced heat pumps at Energy Research and Generation Inc. (ERG) Oakland, Ca., discussed a number of concepts including duplex Stirling and Stirling/Rankine systems. The degree to which these concepts have been reduced to practice was not made clear. It is not known how far ERG has proceeded with hardware development.

Interesting and extensive theoretical studies, reported on a comparative basis, of different heat pump systems including some Stirling engines have been summarized by Wurm and Staats (1977). This work was carried out at the Institute of Gas Technology for the American Gas Association.

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.